This invention relates to the production of aluminum and, in particular, to the recovery of aluminum, oxides, and soluble compounds from saltcake.
Companies producing aluminum can be divided between "primary" smelters producing aluminum from ore and "secondary" smelters producing aluminum from a combination of aluminum ingots and recycled scrap. Secondary smelters melt the aluminum ingots and scrap with a flux consisting of salts, such as sodium chloride, potassium chloride, cryolite (Na.sub.3 AlF.sub.6), or mixtures thereof. The flux melts and floats on the molten aluminum, protecting the aluminum from the oxygen in the air. metallic aluminum combines readily with oxygen, even at room temperature. At the temperature of molten aluminum (1218.degree. F.), the attraction for oxygen is very strong. Even with flux, a substantial fraction of the aluminum reacts to form aluminum oxide. In addition, aluminum globules are entrained in the flux, which is skimmed off as dross.
The dross has an aluminum content of fifteen to fifty percent by weight, the remainder being salts and oxides, and there are many processes for recovering this aluminum. Typically, the dross is heated with additional flux and the aluminum is removed by draining or other means, leaving behind what is known as "saltcake." Saltcake has essentially the same components as dross but in different proportions. Specifically, the recoverable aluminum content is typically four to six percent. At this level, it has not always been economically feasible to further process the saltcake for the recovery of aluminum or the other intrinsic value items, salt and aluminum oxide. The saltcake is raked off, or otherwise removed from, the molten aluminum, stored in suitable containers until cool, and dumped in a landfill as large chunks, e.g. typically twelve to forty cubic feet in volume.
Because of its high salt content, forty to fifty percent by weight, saltcake is ecologically undesirable although not toxic. The problem is that rainwater causes the salt to leach out of the cake, enter the ground, and eventually reach agricultural, industrial, or potable water supplies. A related problem is the shear bulk of the saltcake being sent to landfills. At this time, the amount of saltcake produced in the United States alone is approximately one million tons per year and the rate of production is likely to continue to increase, as it has for the past decade. Reducing or eliminating this material from landfills would significantly ease their burden and would be ecologically beneficial.
While there are commercially viable operations for recovering aluminum from dross, there are few stand alone, commercially viable operations in the world for recovering aluminum, oxides, and flux from saltcake. Part of the problem is the amount of salt, which is approximately half of the weight of the saltcake. Dissolving the salts with water requires large quantities of water and great amounts of energy to heat and then evaporate the water to separate and recover the salts. The cost of the water and energy, compared to the cost of a landfill, is part of the reason presently proposed techniques are not commercially feasible. A landfill is a relatively inexpensive outlet for saltcake.
Another part of the problem is the volume of saltcake to process. Many processes which have been developed in the laboratory or in pilot plants have not proven economically feasible in commercial applications. Increasing the number of facilities is not an answer because of the cost of construction and equipment which must be amortized. Landfills are (presently) an inexpensive alternative, unless salt is banned as a landfill material, as it has been in some areas. Finally, the brine itself is a problem because it is corrosive, eroding and clogging pumps, fittings, and pipes, requiring rigorous maintenance or replacement.
The processes proposed for recycling either dross or saltcake can be divided among those which leach first, those which grind first, and those which leach and grind simultaneously. U.S. Pat. Nos. 4,752,328--Peterson and 4,368,070--Fracchia are examples of the first type; U.S. Pat. No. 4,569,723--Lyon and 4,732,606--Kobele et al. are examples of the second type; and U.S. Pat. No. 4,073,644--Papafingos et al. is an example of the third type.
Leaching first is unsuited to recycling large quantities of saltcake since the chunks of saltcake have a low surface area to volume ratio, causing the leaching to take an unacceptably long time. The Peterson patent describes leaching the salt from one ton of saltcake in a little over nine days. With a million tons of saltcake being produced annually, nine days is an excessive time for processing one ton of saltcake.
Dry grinding is unsuitable since the particles produced are necessarily quite small. Dry grinding is dusty, causes high wear, and is more costly than wet milling.
Basically, all processes use screening to separate the aluminum from the oxides. The underlying assumption in screening is that the aluminum particles are larger than the oxide particles at a particular point in the process.
Dry grinding grinds some of the aluminum into fine particles, which are then collected with oxides and oxidized.
In accordance with the invention, simultaneous grinding and leaching is preferred. It has been found that control of particle/chunk size of the saltcake is important for the recycling process. Unlike the Papafingos et al. patent, the saltcake is in larger chunks going into a mill (four inches vs. less than one inch). Also unlike the Papafingos et al. patent, cold water, not hot water, is used for leaching the salts from the saltcake. The advantages of these, and other, differences from the prior art are discussed in the Detailed Description of the Invention.
In view of the foregoing, it is therefore an object of the invention to provide a process for recycling saltcake that is cost competitive with dumping the saltcake in a landfill.
Another object of the invention is to increase the yield of aluminum in the recycling of saltcake.
A further object of the invention is to provide a process for recycling large quantities of saltcake, on the order of one hundred thousand tons per site per year.
Another object of the invention is to provide a process for completely recycling saltcake into re-usable components, thereby obviating the need to store any residue in a landfill.
A further object of the invention is to minimize the oxidation of aluminum during the recycling of saltcake.
Another object of the invention is to improve the autogenous milling of saltcake.
A further object of the invention is to reduce the wear on pumps and other machinery handling brine.